It is known that piezoelectric materials are characterized by the existence of a vector of polarization which defines an electrical axis and by piezoelectric moduli.
The longitudinal modulus d33 is the constant relation between the density of the electrical charge .sigma. appearing at two electrodes disposed on two faces perpendicular to the electrical axis and a pressure P acting parallel to the electrical axis .sigma. = d33.P.
The transverse modulus d31 is the relation between the density of the electrical charge .sigma.' appearing at two electrodes disposed on two surfaces perpendicular to the electrical axis and a pressure P' acting perpendicularly to the acoustical axis .sigma. = d31.P'.
The hydrostatic modulus dH is the constant relation between the density of the electrical charge .sigma.H appearing at two electrodes disposed on two faces perpendicular to the electrical axis and a hydrostatic pressure H acting on all of the faces .sigma.H = dH.H.
In piezoelectric ceramics of the type of barium titanates and other alkaline earths which are generally employed, the modulus d33 is greater than the modulus d31 and of opposite sign.
The modulus dH is of the same sign as d33 but about five times weaker. In fact, when a hydrostatic pressure is applied, the effects of moduli d33 and d31, of reverse sign, are ignored.
The piezoelectric elements utilized in underwater acoustics generally have the form of discs or of cylinders of revolution whose geometric axis is coincident with the electrical axis carrying two electrodes on their plane faces perpendicular to said axis.
These elements are utilized most often in the longitudinal vibrational mode, that is to say to capture or emit acoustical waves having a direction of propagation parallel to the axis, the lateral faces being then acoutically isolated.
They are also utilized in the transverse vibratory mode to receive or emit acoustical waves propagating perpendicular to the electrical axis, the planar faces being then acoutically isolated.
The sensitivity is thus weaker when the cylindrical transducers with vertical axis are perfectly omnidirectional in bearing.
In order to increase the sensitivity of transducers operating in the longitudinal mode, there have been constructed transducers with pistons composed of one or a plurality of piezoelectric elements of section s interposed between two rigid pistons whose surface S is much greater than the section s. The acoustical pressure which is applied on the external face of the pistons is transmitted through the internal face to the piezoelectric discs and the pressure which is exerted on these along the electrical axis is multiplied by the relation S/s &gt; 1.
Hydrophones with pistons permit improvement of the sensitivity. In contrast, for more elevated frequencies, they are not omnidirectional, which limits their applications.